1. Field of the Invention
The present invention relates to a vehicle steering system that includes a transmission ratio variable mechanism, which changes a transmission ratio between a steering angle of a steering wheel and a turning angle of steerable vehicle wheels.
2. Description of Related Art
In a known vehicle steering system, a transmission ratio variable mechanism is locked by locking a rotatable member, which is rotated synchronously with an electric actuator of the transmission ratio variable mechanism at the time of changing a transmission ratio between a steering angle of a steering wheel and a turning angle of steerable vehicle wheels. In this type of vehicle steering system, the transmission ratio variable mechanism is unlocked by releasing, i.e., unlocking the rotatable member through disengagement of an engaging member, which is engaged with the rotatable member to lock the rotatable member. In Japanese Unexamined Patent Publication Number 2001-48032, the transmission ratio variable mechanism is unlocked by rotating the rotatable member with use of an electric actuator.
Normally, in the vehicle steering system of the above type, a rotational angle of the electric actuator is stored in a storage means (e.g., a memory). Then, the stored rotational angle of the electric actuator is retrieved from the storage means and is used in the control operation of the transmission ratio. Therefore, it is necessary that the storage means is normal to correctly control the transmission ratio. Because of this, for example, a limit rotational angle in a normal operation may be set as a threshold angle. Then, when the rotational angle, which is retrieved from the storage means, exceeds the threshold angle, the storage means may be determined as abnormal. However, this technique may have the following disadvantage. In a particular case where the locking and unlocking of the transmission ratio variable mechanism are repeated, for example, when a rotational direction of the electric actuator is limited to a single rotational direction at the time of checking the unlocking of the transmission ratio variable mechanism, the rotational angle, which is stored in the storage means, may exceed the threshold angle, so that the storage means may be erroneously determined as abnormal.